Method of producing a composite electrode

ABSTRACT

A composite material comprising titanium or a titanium alloy substrate to which is bonded a platinum group metal coating is prepared by a method which includes the steps of wrapping a platinum group metal wire around the substrate, which is in the form of a cylinder, and then converting the wire-wrapped cylinder to a unitary body by annealing and swaging steps. The resultant composite material is particularly useful as an electrode material.

United States Patent Ru in 1451 Oct. 17, 1972 METHOD OF PRODUCING A2,652,621 9/1953 Nelson ..'..29/25.17 COMPOSITE ELECTRODE 2,867,0321/1959 Gehrke et al. ..29/25.l 7 3,165,825 l/l965 Barney ..72/700 X [72]Invent 3,363,304 1/1968 Quinlan ..72/47 x [73] Assignee: EngelhardMinerals & Chemicals 3,426,420 2/1969 Grant et al ..29/474.l X

Corporation I Primary Examiner-John F. Campbell [22] Flled' May 1971Assistant Examiner-Richard Bernard Lazarus [2]] Appl. No.: 146,619Att0rneyMiriam W. Leff and Samuel Kahn 57 ABSTRACT [52] US. Cl...29/25.l8, 29/25.l 1, 29/4733, 1

29 4739 29 4741 29 475 72 47 72 7 A composite material comprisingtitanium or a titani- 51 int. (:1 .1101 9/16, 1101 9/44 alloy Substrateto which is bonded a Platinum 158 Field of Search....29/25.l 1, 25.17,25. 1 8, 473.3, P metal mating is P P by which 29/4739 72/700 47cludesthe steps of wrapping a platinum group metal wire around thesubstrate, which is in the form of a [56] References Cited cylinder, andthen converting the wire-wrapped cylinder to a unitary body by annealingand swaging UNITED STATES PATENTS steps. Theresultant composite materialis particularly f l l t d t 1. 1,700,454 1/1929 Schumacher.;,....29/2s.17 use as 6 W 6 ma em 2,375,068 5/1945 Bennett ..29/474.l X7 Claims, 6 Drawing Figures ag lalialazlallalalalm PATENTEDBBTIT I9123,698,050

SHEET 1 OF 2 FIG.

WIRE WRAP 2 ANNEAL FIG. 3 4

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SWAGE RE-ANNEAL //v l/EN TOP 81/ LEONARD R. RUB/N A T TORNE V METHOD orPRODUCING A COMPOSITE ELECTRODE,

BACKGROUND OF THE INVENTION This invention relates to a method offabricating a composite material comprising titanium or a titanium alloysubstrate and bonded thereto a platinum group metal coating.

Composite materials of this type are well known and find utility in manyfields. They are particularly useful as electrodes in cathodicprotection systems, in electrochemical processes such as theelectrolysis of brine to produce chlorine, or for auxiliarynon-consumable anodes for nickel or chrome plating. The electrodeshavemany configurations, e.g. sheets, wires, rods or tubes, with theplatinum group metal as a continuous layer or a partial or intermittentcoating on the substrate.

Depending on the ultimate use, the physical and chemical propertiesrequired as well as the design of the composite material will vary.However, a requirement of the material which is of major importance inmost applications is that the platinum groupmetal be strongly bonded tothe substrate. It is also important that a minimum amount of theexpensive platinum group metal is used.

Reference to the literature shows that a great many and varied methodshave been utilized to produce the composite materials. Among thewell-known techniques used to apply the platinum group metal arecladding, spraying, electroplating, chemical or thermal reduction,powder metallurgy, and inlay of metal strips. It is known also to followthe deposition of the platinum group metal by various heattreatments,'rolling and swaging, in order to increase the adherence ofthe platinum coating to the substrate. In U. S. Pat. No. 2,719,797, forexample, a platinum coating is applied to a tantalum substrate by theknown methods such as electroplating or chemical decomposition and thebonding is achieved by subjection the coated substrate to a temperatureof 800 C to l400 C in an inert atmosphere. Thereafter the coatedmaterial is drawn out to the desired diameter. Partial coatings havebeen formed according to British Pat. No. 922,599 by rolling into sheetform a mixture of powders of titanium or an alloy of titanium and anoble metal. In another method striped platinum electrodes have beenformed by an inlay technique or by tack or spot welding noble metalstrips to one or more faces of a refractory metal sheet, encasing thesheet with a metal sheath, heating the encased assembly under pressurein an inert atmosphere, and subsequently removing the sheath.

Still other electrodes are composed essentially of loose platinum wirewrapped around a wire substrate, e.g., of titanium coated conductormetal. The platinum wire may be spot welded to the substrate atintervals. These loose wire type electrodes have both mechanical andelectrical disadvantages. The wire is easily torn and the electrodeshave a higher resistivity than electrodes with bonded coatings. If highcurrents are used, passage of the current tends to be mainly at thepoints where the wire is tack welded to the substrate. If gases collectbehind the unbonded wire, the wire tends to lift and the electrodesfail.

The many and varied problems in fabricating suitable composite materialsof platinum group metals on refractory metal substrates are very wellknown to those skilled in the art. However, because of the extensiveapplications in industry of materials of this type, there has been along history of research directed'to finding improved methods ofobtaining the composite materials The primary object of the presentinvention isto provide a simple and economical method for producing acomposite material in which there is a strong bond between the platinumgroup metal coating and the refractory base metal substrate. Anotherobject is to provide a composite material which exhibits structuralintegrity when used as an electrode. A particular object is to providean improved method for preparing a composite material comprised oftitanium or a titanium alloy substrate and a platinum group metalcoating. And still a further object is to provide an improved method forfinding a composite material consisting of a refractory base metalsubstrate with a platinum group metal alloy as the coating. These andother objects are accomplished by the invention described below.

The invention may be best understood by reference to the accompanyingillustrative figures and examples taken in connection with thedescription.

THE DRAWINGS FIG. 1 is a simplified flow diagram of the method of thisinvention.

FIG. 2 is a drawing which illustrates the initial step of wrapping awire around the substrate.

FIG. 3a and 3b are drawings which illustrate two embodiments of thecomposite material which may be formed by the method of this invention.FIG. 3a is a transverse sectional view of a composite material with acontinuous coating and FIG. 3b shows a composite structure with anintermittent coating.

FIGS. 4 and 5 are photomicrographs of samples after the annealing andre-annealing steps, respectively, as described more particularly inExample 2.

The steps outlined in FIG. 1 are explained in detail in the descriptionand examples. In FIG. 2 a platinum group metal wire 10, e.g. of Pt or aPt alloy such as percent Pt-2O percent Ir, is wound around titanium rod11 and secured at both ends 12 and 13. Securing the wire may beaccomplished by many known methods, e.g. by tack welding both ends ofthe wire to the rod or by simply tying the ends through holes drilled inthe ends of the rod. By the annealing, swaging, and second annealingsteps, the wire-wrapped bar is converted to a diffusion bonded unitarycomposite structure which may take various configurations. Twoembodiments are represented in FIG. 3a and FIG. 3b, with the coatingsgreatly exaggerated for the purposes of illustration. FIG. 3a shows acontinuous coating 10a of the platinum group metal on titanium substrate11a, and FIG. 3b shows a composite structure with an intermittentcoating 10b of the platinum group metal on titanium substrate Ilb.

THE INVENTION In accordance with this invention a composite metalmaterial comprising titanium or a titanium alloy substrate to which isbonded a platinum group metal coating is produced by a method whichcomprises:

a. wrapping a wire comprised of a platinum group metal around thesubstrate, said substrate being in the form of a cylinder, to form acontinuous taut helical configuration of the wire around the cylinder,

. annealing the wire-wrapped cylinder in an inert atmosphere at atemperature in the range of about 800 to 1200 C.,

c. swaging the annealed wire-wrapped cylinder at ambient temperature toconvert the wire into' a coating, and

. re-annealing the resultant material in an inert atmosphere at atemperature of about 800 to 1200 C. i

The coating may be any platinum group metal or alloy thereof which canbe made into a wire. Platinum, palladium, iridium, and rhodium, andalloys thereof can readily be drawn into wire. Ruthenium and osmium aretoo brittle to form as fine wire, however, there are alloys of rutheniumor osmium with other metals that are sufficiently malleable to be madeinto suitable wire. It is a particular advantage of this invention thatit is simple to form an alloy coating using the method of this inventionprovided the alloy can be formed as a wire. Examples of particularlyuseful alloy coatings are 80Pt-20lr, 80Pt-20Rh, and 95.5Pt-0.5ThO

The substrate material is titanium or an alloy of titanium that hassimilar expansion characteristics and allotropic modification on heatingas titanium. Titanium has thermal expansion characteristics thatmaterially aid in the developing of a coherent coating by the method ofthis invention. The expansion coefficients of titanium and platinum, forexample, are very close (with that of titanium being slightly greaterthan the thermal coefficient of expansion for platinum), so that thereis no problem of the substrate shrinking away from the platinum groupmetal wire or coating when it is heated. In addition an allotropicmodification takes place in titanium at 880 C with a 5 percent increasein volume. Thus when the wire-wrapped titanium is heated above thistemperature there is a resultant tightening action on the wire that aidsin developing contact for diffusion bonding. It is noteworthy thattantalum and niobium, which do not have comparable thermal coefficientsof expansion as titanium and are not subject to the same allotropicmodification do not.

form a cohesive bond with a platinum group metal when subjected to thesame steps of preparation described herein.

In accordance with this invention there are two annealing steps, one inwhich the platinum group metal wire is diffusion bonded to the substrateand the second in which the platinum group metal, now converted to acoating, is diffusion bonded to the substrate. In both these steps thediffusion bonding is aided by the allotropic modification of thetitanium when it is heated. Thus the annealing steps are preferablycarried out at a temperature above 880 C. Annealing is performed in aninert atmosphere, i.e. an atmosphere which will not harm the titaniumsubstrate. Oxygen or nitrogen, for example, has a harmful effect ontitanium at elevated temperatures. Annealing can be performed, forexample, in a vacuum furnace or in the presence of an inert gas such ashelium or argon.

It was noted above that the platinum coating may be continuous orintermittent. Depending on the design requirements, the wire is chosenof suitable diameter and length to give the thickness and coatingdesired. The dimensions of wire may vary considerably and can be easilycalculated. For example, when a 3 mil platinum wire, which has a crosssection of 7.07 square mils, is wound on a 7.07 mil pitch on a inchdiameter titanium rod, the length of wire required to form a 1 mil thickcoating is about 1 l .74 feet of wire per square inch of rod surface. a

An alternative approach is to use a ribbon or metal foil having athickness of about 1-3 mils. This approach entails handling difficultiesand higher labor costs. In the preferred method of this invention, thesubstrate is wrapped with wire, which is, in essence, formed into aribbon in the swaging step. In contrast to ribbon or foil, wire is easyto handle, and generally readily available commercially.

Before wrapping the wire around the substrate, the substrate materialshould be suitably cleaned. The usual methods of cleaning the titaniumor titanium alloy surface may be used. For example, the surface may becleaned with emery paper and acetone or it may be subjected to an acidetch. It is an advantage of the process of this invention that thecleaning of the substrate is not critical. A simple cleaning of thetitanium surface with emery paper and acetone has proved sufficient.

The wire is wrapped around the cylindrical substrate at preferably ahigh degree tension to form a tight coil, preferably near the yieldpoint of the wire, and it is secured at both ends of the cylindricalsubstrate. The substrate is in the form of a cylinder since the wirewill make even contact at all points on this cylindrical configuration,thus enhancing diffusion bonding of the wire to the substrate. lt ispossible for the substrate to be a hollow cylinder.

During the swaging, the wire is flattened on the substrate. This step isperformed at ambient temperature to avoid oxidizing the titanium. It isstill another advantage of this process that the coating can be formedat room temperature. During this swaging step, the substrate may bereduced in diameter.

After the second annealing step the composite material may be furtherreduced in diameter or converted to another configuration, e.g.round-cornered square rod, flat stock, channels, ells, tees, and thelike.

EXAMPLE 1 A sample of inches diameter titanium rod was cut to anapproximately 10 inch length to allow chucking in a lathe. An adaptorwas built to hold a spool of 3 mil platinum wire in the tool holder ofthe lathe and to apply enough friction to the spool to keep the wirerelatively taut when being paid out. The running tension on the wire wasmeasured. On 3 mil wire, the drag is approximately grams, whichgenerates a stress level of about 22,000 psi in the wire while it isbeing paid out.

The rod was rubbed with emery cloth to remove surface contamination andstarting and finishing holes were drilled for the wire. it was wiped offwith acetone soaked rags to remove handling contamination and thenchucked into the lathe. The wire was passed through the starting holeand tied off, The feed on the lathe was set at 7.1 mils/revolution andthe wire paid off on to the rod. At the end of the run, the wire wascut, passed through the finishing hole, and tied off.

The wire-wrapped rod was annealed for 1 hour in vacuum at 950 C. Thistemperature wasvchosen to take advantage of the allotropic modificationwhich takes place in titanium at 880 C with a 5 percent increase involume. The resultant tightening action on the wire aids indevelopingcontact for diffusion bonding.

The wire-wrapped rod was then swaged without lubricants to a diameter of0.365 inch including the wire wrapping. The turns of wire had flattenedon the surface until they butted against each other, forming arelatively continuous sheet. After swaging the coated rod wasre-annealed'for 1 hour at 950C in vacuum, completing the diffusionbonding.

In the resultant composite material, the platinum coating which wasessentially continuous was firmly bonded to the titanium substrate.

A composite sample prepared in this manner was bent to an angle ofapproximately 90 on a 1.5 inch radius. The coating remained firmlyadhered to the substrate. This demonstrates the tenacity of the bond.

EXAMPLE 2 Several samples of the titanium rod were coated with platinumusing the procedure in Example 1. Someof the samples were used formetallographic examination at various stages of the process.

FIG. 4 is a photomicrograph at a magnification of about 500 times of across section through a single turn of platinum wire on a titanium rodafter wire wrapping and an annealing treatment at 950 C for 1 hour in avacuum, but prior to swaging. It shows that diffusion bonding hadoccurred between the Pt wire and the Ti rod.

FIG. 5 is a photomicrograph at a magnification of 200 times of a sectionthrough the surface of a platinum coated titanium rod after wirewrapping, annealing, .swaging, and re-annealing. Both annealing andre-annealing wire performed at 950 C for 1 hour in a vacuum. It showsthat diffusion bonding had occurred between the Pt coating, which isessentially continuous, and the titanium substrate.

EXAMPLE 3 Several samples of titanium rod were coated with platinumusing essentially the same procedure as Example l, but with thevariation described below.

The titanium rods were cleaned as described in Example l,pre-coated'with a thin film of platinum by deposition from anorgano-metallic compound of platinum, and then wire-wrapped as describedin Example l. As in the samples prepared in Examples 1 and 2, all ofthem were wire-wrapped without difficulty or incident. I

As an alternative procedure, some Pt wire-wrapped titanium rods,prepared as otherwise described in Example 1, were annealed at 800 C.Because of the close thermal coefficients expansion of platinum andtitanium, the bonding was adequate. However, it is preferred to annealabove 880 C to cinch the wire in case contact was marginal. In thesecond annealing step after being swaged these samples were subjected totemperatures of 950 C in a vacuum to insure diffusionbonding.

EXAMPLE 4 Niobium and tantalum rods were wire-wrapped with platinum wireand subjected to annealing temperatures of 950 C and swaged, in aprocedure similar to that described in Example 1.

Theplatinum wire would not adhere to either materialaft'er wrapping andvacuum annealing for an hour at 950 C. Diffusion studies have shown thathis extreme-.

ly difficult to make platinum and tantalum stick to each other, evenwhen metallographically polished slugs are EXAMPLE 5 The surface of acomposite platinum on titanium rod prepared as described in thepreferred method of Example l was covered with Platers tape, leaving aV2 inch diameter circle of platinum exposed. The tape portion wasimmersed in a beaker of reagent grade HCl (37 percent l-lCl) as an anodeat 5 amperes for 30 minutes (3670 amperes/ftF). (Normal current densityin service is about 200 amperes/ft?) This half-hour test is enough toremove completely a l50 microinch platinum coating deposited by aplating technique. The test was repeated 7 times without change inovervoltage. The test was discontinued during the 8th cycle because of arise in voltage above the 800 millivolts considered allowable inservice. The coating was found to have failed by .delamination due toattack on the swaged wire at the tape-wire-electrolyte interface, whichis the locus of a current discontinuity not found in actual service.

EXAMPLE 6 A 0.375 inch diameter titanium :rod was wire wound with 3 milPt wire, diffusion bonded at 950. C in vacuum, swaged to 0.365 inchdiameter including wire, finish bonded, and rolled with two intermediateanneals at 950 C to 0.131 inch thickness without visible damage to thecoat. A titanium pipe, 0.540 inch O.D. X 0.065 inch wall was wire wound,swaged to 0.500 inch diameter, finish bonded, and rolled to 0.128 inchthickness without incident, difficulty, or visible surface damage. Thefabrication of these two flat pieces has demonstrated that the wirewrapping technique is not limited to designs requiring cylindricalsymmetry. One rod was bent at right angles in the coated area withoutapparent damage.

Coated rods were also easily fabricated using the technique of theinvention in such a manner as to supply only an intermittent surfacecover to the titanium substrate. Such spacing at intervals has beenshown to be desirable in the design of certain types of electrodes usedfor cathodic protection.

What is claimed is:

1. A process for producing a composite metal material comprisingtitanium or a titanium alloy substrate to which is bonded a platinumgroup metal coating which comprises the steps of:

a. wrapping a wire comprised of a platinum group metal around thesubstrate, said substrate being in the form of a cylinder, to form acontinuous taut helical configuration of the wire around the above about880 C.

3. A process according to claim 1 wherein annealing and re-annealing isperformed at a temperature above 880 C.

4. A process according to claim 1 wherein the wire is composed of Pt,Pd, Rh, or Ir.

5. A process according to claim 1 wherein the wire is an alloy of aplatinum group metal, said alloy being formable into a wire.

6. A process according to claim 1 wherein the resultant platinum groupmetal coating is a continuous layer on the substrate.

7. A process according to claim' 1 wherein the resultant platinum groupmetal coating is an intermittent layer on the substrate.

2. A process according to claim 1 wherein the resultant material is re-annealed at a temperature above about 880* C.
 3. A process according to claim 1 wherein annealing and re-annealing is performed at a temperature above 880* C.
 4. A process according to claim 1 wherein the wire is composed of Pt, Pd, Rh, or Ir.
 5. A process according to claim 1 wherein the wire is an alloy of a platinum group metal, said alloy being formable into a wire.
 6. A process according to claim 1 wherein the resultant platinum group metal coating is a continuous layer on the substrate.
 7. A process according to claim 1 wherein the resultant platinum group metal coating is an intermittent layer on the substrate. 